The present invention relates to a device for the separation of airborne particles, such as those in an aerosol, into grain size classes.
Devices of the above indicated type are instruments which separate the particles still in suspension into grain size classes and collect them, whilst maintaining the separation, on a filter. It is then possible to perform a series of chemical and physico-chemical analyses on the deposited particles in dependence on the dimensions of these for the purpose of determining the risk resulting from inhalation of dust in the environment in which they are present. The said devices are therefore utilized in the testing of environmental and industrial health, in medical physics and in powder technology generally.
The devices currently in the market in the United States for commercial usage, consist, schematically, of a rectangular channel having a substantially L-shape configuration, traversed by filtered air. This channel then has two parts the first of which is defined by a first nozzle which ejects filtered air into the second part which is delimited above by a body and below by a support plate on which a filter is positioned. Within the first nozzle there is positioned a second nozzle which ejects the particle-bearing air such as an aerosol. The introduction of the aerosol takes place therefore upstream of the curvature of the said channel. This means that as the particles travel past the curved part of the channel they are separated in dependence on their aerodynamic dimensions into various streams. Each stream is composed of particles of the same aerodynamic diameter. Such streams are subsequently deposited on filters starting from the stream with the particles of greater diameter. The support plate is rectangular and is made in stainless steel. This plate is supported by a pipe union, and, more precisely, rests on a flange extending from the said pipe union.
The devices described above have various serious disadvantages.
In particular, since the plate rests freely on the flange of the pipe union, and since this, being of reduced dimensions, is difficult to work and therefore has certain tolerances, there is obtained an air space between the plate and the body of different thickness which prejudices the correct operation of the device, and it can happen that an irregular deposition of the particles of the aerosol takes place. In fact, for a correct operation of the device, it is necessary that the channel delimited by the plate and by the said body should have a constant height. The plate must therefore be edgewise with the edge of the flange and must be flat. But, since the sintered plate is made of stainless steel, grinding to make it coplanar is difficul to do in that the material of which it is made has a certain degree of elasticity. This involves the possibility of differentiated deposits of particles on the filter. It is also necessary to note that setting up of the plate onto the flange is manual and this can involve difficulty and positioning errors of this plate. Finally, the fact that the material of which the plate is made has characteristics of elasticity can involve buckling of this towards the interior of the pipe union because of the pressure difference existing between the interior of the channel and the interior of the pipe union.